When someone registers they are considered as "New Members" or what ever you used as a new group. You would set that one as auto subscribe and a rank to that one which they will get when they register. Every other rank must be manually done in the Admin Panel, just like Base said on Top.

...i created a Group named "Audience"and"VIP members". this 2 Groups has users with 0 and 1 or more posts.....but the first registering user is in Audience....but when i promote this person from Audience to VIP then he is prmoted .but the rank is audience...that i do it manually to VIP users..is there any way to make it automatically....

and also when a VIP user with 1 or 2 post make a thread or post....then he is automatically aded to Audience group....and he is then belonging to That 2 Group...is there any way to stop a VIP member from putting in Audience Group